The most common and most likely answer to this crossword clue is the 10 letter word DUNDERHEAD
I've seen this crossword clue in The Times
D
U
N
D
E
R
H
E
A
D
More clues leading to the result DUNDERHEAD
Buddy explains!
The straight part
We know that "D for dimwit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case d for dimwit = dunderhead.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DUNDERHEAD mean?
DUNDERHEAD noun- (somewhat obsolete) A stupid person; a dunce.